In conventional overhead valve internal combustion engines, at least two valves reciprocate to provide intermittent communication between intake and exhaust manifolds and a combustion chamber. The valves include valve stems that are commonly disposed in valve stem guides, supporting axial motion in an engine component such as an engine head. Lubrication is provided to upper portions of the valve stems by a spray of lubricating oil within a valve cover disposed over the engine head or by gravity flow from an associated rocker arm. Oil flows along a free upper end of the valve stem toward the manifolds and valve heads by the force of gravity and may be encouraged by a pressure differential in the manifold versus crankcase pressure.
Annular valve stem seals are generally urged into contact with the outer surface of the valve stem and an upper portion of the valve guide by a valve stem seal retainer, and serve various purposes. First, valve stem seals minimize engine oil consumption by restricting oil entry into the manifold and the combustion chamber. Second, they help to minimize exhaust particulates that contribute to pollution. Third, they are helpful in minimizing guide wear, which is of particular importance in large diesel engines due to the nature of their operation. The valve stem, valve guide, and valve stem seals are annularly wrapped by a helical compression valve spring that serves to bias the valve into a closed position. The longitudinal ends of the valve spring are restrained by flanges on corresponding valve spring retainers and/or spring seats, thereby maintaining proper alignment and position of the valve and valve spring.
In the heavy-duty engine market a number of changes are being made to comply with recent and prospective emissions standards. As the construction of the engine changes, engine designers must nevertheless maintain a robust engine design with a sufficient level of dependability. One of the more prominent changes being implemented is the increase of the power rating of the engine in an effort to reduce the size of the engine. In particular, engine manufacturers are attempting to reduce the displacement of heavy-duty engines while still providing ample horsepower and torque for heavy-duty applications. As is well-known, engine displacement is calculated by multiplying cylinder bore area times the piston stroke length. In reducing the displacement of heavy-duty engines, manufacturers are reducing both the bore area and the stroke length while increasing the compression within the combustion chamber. Increasing the required amount of compression, in turn, places greater stress on the valve seal. Many of these engines are increasing their compression by up to 50-60 psig, which is a far greater pressure than many prior art valve seals can handle while being properly retained on a valve guide. For such cases, an integral valve seal with a metal retainer is normally recommended.
However, as the bore area of an engine is reduced, the area provided for valve assemblies above a combustion chamber is correspondingly reduced. The problem is especially significant in heavy-duty diesel engines because all valve assemblies are typically oriented perpendicular to the engine head. Additionally, a fuel injector occupies a large portion of the area above the cylinder bore. Thus, in high efficiency heavy-duty diesel engines having more than two valves (intake and exhaust valves) per cylinder, the area directly above the engine bore must be shared by a fuel injector and the valves. Since the size of the fuel injector is substantially fixed, a reduction in engine bore generally requires a reduction in the valve assembly diameter, including corresponding reductions in the diameter of valve stem seals, valve guides, and valve stem seal retainers. There is thus a need for a valve seal assembly capable of withstanding increased compression loads while providing a seal having close clearance and durability.